This invention relates generally to control of traffic signals under emergency conditions and, more particularly, to systems for automatic control of traffic signals in such a way as to preempt normal signal operation and permit the unimpeded and safe passage of emergency vehicles, such as ambulances, police cars, fire trucks and so forth. There is a related need for preemption control systems to give priority to non-emergency vehicles, such as buses, which have to maintain a schedule in busy traffic. It has been widely accepted that, particularly in high density traffic areas, there is a vital need for such systems. Not only must an emergency vehicle be free to move through an intersection in a selected direction of travel, but the traffic flow in other directions, which could interfere with the movement of the emergency vehicle, must be stopped by an appropriate signal indication until the emergency vehicle has passed.
A number of present day systems have been utilized to accomplish this general purpose, but all have fallen short of the desired result in some respect. A common approach has been to provide apparatus on the emergency vehicle to transmit an emergency signal to a receiver associated with the traffic signal. The traffic signal controller, which has been suitably modified, is actuated to operate the signals in a predetermined emergency sequence. Some systems of this type require the installation of a receiver or sensor under the road surface, to detect where the vehicle is located with respect to the traffic signals. Obviously, systems of this type present difficulties of installation and maintenance. Various traffic preemption systems use radio signals, infrared signals, ultrasonic signals, audio signals or optical signals transmitted from the emergency vehicle and detected in some manner at the controlled intersection. A common difficulty with all systems of the prior art is that of accurately determining the time of arrival of the vehicle at the intersection. Clearly, preemption of normal traffic control should not occur too early. Apart from the obvious inefficiency this entails, there is an element of risk in that impatient drivers may try to enter a controlled intersection before the emergency vehicle arrives. Various techniques have been proposed for determining vehicle location for use in traffic preemption. For example, electronic "signposts" can be installed beside or beneath the roadway to detect the passage of vehicles. Such systems are used, for example, to determine the locations to help maintain more accurate bus schedules. For the traffic preemption problem, however, installing multiple sensors or "signposts" near each intersection would be very expensive and still would not necessarily provide the desired accuracy.
More specifically, optical preemption systems are limited by the line of sight between the vehicle and the intersection control unit. Audio preemption systems detect the sound of an approaching siren on an emergency vehicle and take appropriate action. Unfortunately, the sound can be blocked by other vehicles or buildings, and microphones must be installed at points approaching the intersection.
Radio preemption systems currently available utilize signal strength to determine distance from the intersection. However, natural variations in terrain and man-made obstructions render this approach quite inaccurate.
Beacon based systems are more accurate, but do not permit subsequent adjustment to preemption distance needed for changing traffic patterns or construction zones. Also the installation cost of a beacon system is high because long lengths of underground cable have to be installed beneath the intersection and its approaches. Sensor loops underground are used to sense the vehicle positions, but are prone to breakage in cold weather. A failed sensor can render the system inoperative while repairs are made over an extended period, possibly months.
Another common aspect of traffic signal preemption systems is that they are typically manually actuated from the emergency vehicle. When the driver actuates a button or switch in the vehicle, an emergency signal is transmitted to the controller at the intersection, to preempt normal operation and modify the controller temporarily for passage of the vehicle. Some systems allow the driver to indicate a direction of turn at the intersection, so that the traffic signals can be appropriately controlled during preemption. However, existing preemption systems are typically not optimized to disrupt normal traffic control for as short a time as possible, or to clear as much of the interfering traffic as possible from the intersection. Moreover, existing preemption systems have no provision for preempting signals at adjacent intersections to one side or the other of the vehicle direction of travel when a preemption request is made. In existing systems, some intersections may not be preempted soon enough, if the vehicle deviates from a straight path along a single street, and may be preempted unnecessarily if the vehicle turns before reaching a nearby preempted intersection. The present invention has the goal of providing safe and unobstructed passage for emergency vehicles, while at the same time minimizing disruption of normal traffic through the intersection, and eliminating many of the disadvantages of the prior art systems.